In optical network test and measurement applications, it is important to accurately control multimode launch conditions from a light source for the purpose of improving consistency of fiber attenuation measurements, whether in an LED light source application or an OTDR application.
A problem that arises is controlling the launch condition from a multimode source, whether it is an LED source or a laser source, such that the number of mode groups is launched with a high degree of precision. Controlling the mode groups in multimode fiber is the key to making repeatable, accurate, and consistent loss measurements with a light source and power meter.
The most common method for controlling launch conditions is to use a mandrel wrap of specified diameter and number of turns. With reference to FIG. 1, a view of a mandrel, the mandrel 12 is configured to receive multiple wraps of fiber 14. This method will strip out the loosely coupled higher order modes that cause attenuation measurement inconsistency. However, this method does not provide consistent loss measurements from light source to light source because each light source has a unique launch condition depending on how a fiber is coupled to the source and the manufacturer of the source. Variations as high as 50% are possible. With emerging standards requiring variation reductions to 10%, such as Encircled Flux (EF), a new method for controlling launch conditions is needed.
A second problem that arises is providing a source that can accommodate different multimode fiber sizes and different wavelengths. It is advantageous for the customer to use one light source that can be used with 50 μm fiber, for example and have a controlled launch condition that is satisfied for two wavelengths, such as 850 nm and 1300 nm, from the same launch cord.
Previous EF compliant mode controllers were meant to be universal such that they can be used with any LED light source. However, these devices are large, heavy, and difficult to use and manufacture.
A problem that arises in connection with OTDR is insertion loss at the output of the device. While a certain amount of loss is expected between the output and input of the device, it is important that the loss remain at a “reference grade termination”. A reference grade termination is one that achieves a loss of less than 0.1 dB. Per standards requirements, a reference grade termination is required at interface connections of launch cords.
Previous art for controlling the launch condition of a multimode source was to wrap the test cord around a mandrel of specified size, with 5 turns for example, as in FIG. 1. This mandrel required a circular device of fixed outside diameter, such as 20 mm, unto which the fiber cable was wrapped. This configuration creates a stacking effect, which causes the mandrel to increase in height.